A shift fork used to move a coupling sleeve in an axial direction when a transmission gear shift is performed, is manufactured for example by aluminum diecasting. During manufacture, an engaging part which engages with an engaging groove of the coupling sleeve is formed at the tip of the fork main body.
Conventionally, ADC12 was chiefly used as a diecasting material in aluminum diecasting. Recently, high silicon aluminum diecasting materials with superior wear-resistant properties to those of ADC12 have also come into use. Such materials are used to mold the fork main body and the engaging part in a one-piece construction so as to decrease costs and achieve a lightweight construction.
As shown in FIG. 5, a claw-shaped engaging part 52 is usually molded integrally at the tip of a fork main body 51, and the engaging part 52 slides on a rotating coupling sleeve 54. In order to obtain high wear-resistant properties in the engaging part of high silicon aluminum shift forks, a casting surface is cut by 50 .mu.m or more so as to cause pro-eutectic silicon to form on the surface.
FIG. 6 is a view from the direction of the arrow VI in FIG. 5. Flat engaging surfaces 53L, 53R are formed on the engaging part 52 by the above process.
However, in the prior art shift fork, because there is a space between the engaging part 52 and coupling sleeve 54, when the shift fork bends and the engaging part 52 inclines, the engaging surfaces 53L, 53R and coupling sleeve 54 do not come into contact evenly. For instance, when the shift fork 50 moves to the right of FIG. 6, the rotating coupling sleeve 54 and the upper right corner of the engaging part 52 will come in contact. As a result, the surface pressure increases, the wear of the engaging part 52 increases, and the engaging part 52 wears unevenly as shown by the dotted line in the figure.
As shown in FIG. 5, ideal points P.sub.1L, P.sub.1R, at which the coupling sleeve 54 should be pushed, lie on a straight line which passes through the center O of the coupling sleeve 54. However, when the engaging part 52 inclines and comes in contact with the coupling sleeve 54, the positions at which the engaging part 52 pushes the coupling sleeve are points P.sub.2L, P.sub.2R which are offset upwards in the figure above the ideal points P.sub.1L, P.sub.1R.
When the contact point of the engaging part 52 and coupling sleeve 54 shifts from the ideal points P.sub.1L, P.sub.1R in this way, a force acts on the coupling sleeve 54 in a direction other than axial and operating performance decreases.
Tokkai Hei 5-340471 issued by the Japanese Patent Office in 1993 and Tokkai Hei 6-117451 issued in 1994 disclose a method regarding wear-resistant properties of the engaging part wherein a wear-resistant member is made to adhere to the engaging part, and deformation of the engaging part is prevented.
Further, Tokkai Hei 8-49035 published by the Japanese Patent Office in 1996 discloses an engaging part integrally molded with the fork main body from an wear-resistant aluminum alloy, and shot peening is performed.
However, in all these cases, additional machining is necessary after diecasting.